1. Field of the Invention
This invention relates to urea-zinc oxide fertilizer compositions and methods of preparing them. More particularly, this invention relates to urea-zinc oxide particles having zinc oxide homogeneously dispersed throughout the urea particles and to a process of preparing such particles.
2. Description of the Prior Art
Urea is widely used in many fertilizer applications as a source of nitrogen. In many instances, it is the preferred nitrogen source, since it contains a higher percentage of nitrogen as compared with many other available nitrogen-containing materials. Furthermore, urea may be made into a variety of particulate forms, such as granules, crystals, or prills, which are convenient to handle.
Zinc has been used with a variety of fertilizer compositions as a desirable trace element. However, in many applications, especially for the growth of rice, corn or the like, zinc is often used in relatively large amounts on the order of 8%. Heretofore, however, no suitable method has been devised for incorporating such relatively large amounts of zinc into urea fertilizers. In the past, zinc compounds have generally been applied as a separate step in fertilizing a field, which, of course, is highly undesirable from the labor standpoint.
Prior art methods for adding zinc salts to fertilizer compositions for one-step application included dry blending, coating, and solution dispersion methods. Two major disadvantages detract from all of these methods, viz., hygroscopicity and the difficulty of obtaining a uniform product.
Dry blending of zinc salts with particulate fertilizer compositions is generally unsatisfactory, since combinations of zinc salts and urea are usually hygroscopic and thus absorb moisture from the air and agglomerate into an unusable solid cake. Furthermore, powdery zinc compounds readily separate from the particulate material, resulting a waste of zinc, often interfering with the proper operation of fertilizer dispensing machinery, and causing nonuniform application to crops.
In one attempt to overcome such disadvantages, several prior art methods of coating zinc compounds on particulate fertilizer compositions have been described. However, the inherent highly hygroscopic properties of the zinc salts still cause problems, since particulate fertilizer compositions coated with these zinc salts tend to absorb moisture from the air and agglomerate into an unusable solid cake. Attempts to slow the rate of water absorption by the use of a clay binder or other exterior coating have not been sufficient to counteract the increased rate of water absorption. Furthermore, it is extremely difficult to obtain uniform coating of zinc salts on urea. The presence of an uneven coating results in exposed areas which provide sites for the urea particles to cake or stick together, and the zinc compounds tend to powder off from the urea particles, forming a zinc powder separate from the particulate material to be applied. Thus, the disadvantages of the dry blending technique are not overcome in a wholly satisfactory manner. A further disadvantage of prior art coating processes is the wide variation in results which are obtained from zinc compounds purchased from different manufacturing sources.
Dispersion methods for adding zinc compounds to particulate fertilizers also pose problems due to the hygroscopic properties of the zinc additives. These properties are particularly troublesome in causing dissolution and/or caking of free-flowing, particulate urea compositions; since urea must have a low water content to remain free-flowing and have good storage characteristics. This level of dryness is difficult to achieve after highly hygroscopic, water-soluble zinc salts such as zinc sulfate, zinc nitrate, and zinc chloride have been added to a urea melt.
While initially it had been thought that water-soluble zinc salts had to be used in urea fertilizers, recent reports have demonstrated that water-insoluble zinc compounds are satisfactory for many applications. Accordingly, it was thought to add certain water-insoluble zinc salts to urea, but these salts tended to be difficult to disperse, resulting in an uneven distribution and uneven release of zinc into the soil.
Those concerned with the development of solid, particulate, zinc-containing fertilizer compositions have long desired a particulate urea-zinc fertilizer which could be handled in the same manner as particulate urea fertilizers. Furthermore, it would be highly desirable if such particulate fertilizers could be prepared by a relatively simple process which did not involve coating the zinc material onto the urea particles and did not significantly increase the hygroscopicity of the resultant product. The present invention fills such needs.